Using Self‐Assembling Dipole Molecules to Improve Charge Collection in Molecular Solar Cells

Abstract

Surface modification of indium tin oxide (ITO)‐coated substrates through the use of self‐assembled monolayers (SAMs) of molecules with permanent dipole moments has been used to control the anode work function and device performance in molecular solar cells based on a CuPc:C60 (CuPc: copper phthalocyanine) heterojunction. Use of SAMs increases both the short‐circuit current density (Jsc) and fill factor, increasing the power‐conversion efficiency by up to an order of magnitude. This improvement is attributed primarily to an enhanced interfacial charge transfer rate at the anode, due to both a decrease in the interfacial energy step between the anode work function and the highest occupied molecular orbital (HOMO) level of the organic layer, and a better compatibility of the SAM‐modified electrodes with the initial CuPc layers, which leads to a higher density of active sites for charge transfer. An additional factor may be the influence of increasing electric field at the heterojunction on the exciton‐dissociation efficiency. This is supported by calculations of the electric potential distribution for the structures. Work‐function modification has virtually no effect on the open‐circuit voltage (Voc), in accordance with the idea that Voc is controlled primarily by the energy levels of the donor and acceptor materials.

Influence of Molecular Aggregation on Electron Transfer at the Perylene Diimide/Indium-Tin Oxide Interface

“…Transparent conductive oxides (TCOs) are essential electrode materials for applications in organic electronics, such as organic photovoltaics (OPVs), [1][2][3][4] organic thin-film transistors, 5 organic light-emitting diodes, 6,7 and perovskite solar cells. 8 Understanding and controlling the surface properties of TCO electrodes can improve the performance of these types of devices.…”

“…The enhancement was attributed to a greater short circuit current (J SC ), courtesy of an enhanced interfacial electron-transfer (ET) rate at the anode resulting from a shift in the ITO work function, which lowered the injection barrier between the electrode Fermi level and the CuPc HOMO. 2 Redox-active modifiers offer the possibility of further enhancing the charge-collection efficiency at a TCO/organic interface by serving as a "mediator" layer with a tunable redox potential that can be matched to the relevant charge-transport energy levels. 3,15 Lin et al 15 used redox-active zinc phthalocyanine phosphonic acids (ZnPcPAs) to modify ITO electrodes that were tested as bottom contacts in CuPc/C 60 planar heterojunction solar cells.…”

“…Furthermore, PDI redox potentials can be tuned, most effectively through substitutions at the "bay" (1,6,7,12) and/or "ortho" (2,5,8,11) positions, allowing them to be energetically matched to different active-layer and TCO materials. 25,26 In a recent study, 27 we began examining the properties of PDI monolayers tethered to ITO via phosphonic acid (PA) anchoring groups.…”

A PbS nanocrystal-C60 photovoltaic device for infrared light harvesting

“…There are, therefore, two possible explanations for the increase in V oc upon annealing of the nc-PbS layer: It is plausible that the ITO/nc-PbS contact is modified upon removal of the oleic acid ligand, thereby improving alignment between the ITO Fermi level and the relevant transport band in the adjacent nc-PbS. Alternatively, since it is well known that organic adsorbates can affect the work function of inorganic surfaces by modifying the surface potential, 23 it is possible that the offset between the nc-PbS valence band and C 60 LUMO is reduced in the presence of oleic acid ligands, thereby reducing V oc .…”

Water-soluble multiwall-carbon-nanotube-polythiophene composite for bilayer photovoltaics

“…It has recently been shown that V oc in bilayer organic solar cells is virtually independent of the work function of the hole-extracting electrode. 10 In this case the work functions of the microwave oxygen plasma treated ITO electrode and o-MWCNTs were measured using ultraviolet photoelectron spectroscopy ͑UPS͒ to be ϳ4.4 and ϳ5.0 eV, respectively. Such a small change in V oc would be consistent with this idea.…”